1. Field of the Invention
The invention relates to the field of hearing aids. The invention, more specifically, relates to a hearing aid having an adaptive filter for generating a feedback cancellation signal, to a method of reducing acoustic feedback of a hearing aid and to an electronic circuit for a hearing aid.
2. The Prior Art
Acoustic feedback occurs in all hearing instruments when sounds leak from the vent or seal between the earmould and the ear canal. In most cases, acoustic feedback is not audible. But when in-situ gain of the hearing aid is sufficiently high, or when a larger than optimal size vent is used, the output of the hearing aid generated within the ear canal can exceed the attenuation offered by the earmould/shell. The output of the hearing aid then becomes unstable and the acoustic feedback becomes audible, e.g. in the form of a whistling noise. For many users, and for the people around, such audible acoustic feedback is an annoyance and even an embarrassment. In addition, hearing instruments that are at the verge of feedback, i. e. sub-oscillatory feedback, may influence the frequency characteristic of the hearing instrument and lead to intermittent whistling.
WO-A1-02/25996 shows a hearing aid with an adaptive filter to compensate for the feedback. The adaptive filter estimates the transfer function from output to input of the hearing aid including the acoustic propagation path from the output transducer to the input transducer. The input of the adaptive filter is connected to the output of the hearing aid and the output signal of the adaptive filter is subtracted from the input transducer signal to compensate for the acoustic feedback.
The adaptive acoustic feedback cancellation systems as described above allow a substantial suppression of acoustic feedback, thereby allowing an increase of 10 to 12 dB of usable gain, as is e. g. described in Kuk, Ludvigsen and Kaulberg, “Understanding feedback and digital feedback cancellation strategies” in The Hearing Review, February 2002, available at http://www.hearingreview.com/Articles.ASP?articleid=H0202F04. This article also gives a comprehensive overview of the phenomenon of acoustic feedback with hearing instruments and strategies to suppress this feedback.
Nevertheless, there remain problems associated with adaptive feedback cancelling. The correlation analysis is performed to estimate the feedback path. This is based on the assumption that a feedback signal is a highly correlated version of the original signal. If higher correlation is observed, but the duration of the correlation analysis is short, the system may suggest the presence of feedback when actually no such feedback has occurred. This is an artifact of the feedback analysis algorithm. In real-life, most speech and music signals are highly correlated on short-term basis but not on a long-term basis. Thus, short-term correlation analysis on speech and music could result in cancellation of some signals, and could even lead to unpleasant sound quality and loss of intelligibility. This suggests that long-term correlation (i.e. slow feedback path estimation) should be used to avoid such artifacts.
On the other hand, if the feedback cancellation algorithm takes a long time to cancel the feedback signal, it may not be able to handle sudden changes in the characteristic of the feedback path. Audible feedback may still result until the feedback cancellation algorithm has successfully estimated and cancelled the feedback signal. Thus sudden changes, e.g. placing a telephone handset next to the ear, will result in whistling that may last several seconds before the feedback cancellation algorithm is effective in suppressing the annoying signal. This is undesirable and the successful algorithm should (ideally) handle sudden changes in the feedback path.
Moreover, the feedback cancellation algorithm may have different effectiveness in different frequency regions, i. e. provide an adequate feedback suppression in one frequency band while producing undesirable results in other frequency bands.
A further problem in the case of a relatively slow adaptation time constant occurs if a high-feedback environment suddenly changes into a low-feedback environment, e. g. if the hearing aid wearer puts back a telephone handset. The adaptive filter then subtracts (adds after inversion) from the signal path a strong feedback cancellation signal which no longer is needed for signal cancelling. In this case the adaptive filter actually generates a whistling sound rather than removing one. Acoustically this sound is indistinguishable from the sound of feedback, and therefore it is in common language referred to as feedback, although it would be more correct to say that it is due to the attempt by the adaptive filter to create a feedback cancellation signal.